Data Sheet AD7475/AD7495
Rev. C | Page 19 of 24
POWER VS. THROUGHPUT RATE
By using the partial power-down mode on the AD7475/AD7495
when not converting, the average power consumption of the ADC
decreases at lower throughput rates. Figure 24 shows how, as the
throughput rate is reduced, the device remains in its partial power-
down state longer and the average power consumption over
time drops accordingly.
THROUGHPUT (kSPS)
100
0.001
0
POWER (mW)
50 100
0.01
0.1
1
10
150 200 250 300 350
AD7495 5V
SCLK = 20MHz
AD7495 3V
SCLK = 20MHz
AD7475 5V
SCLK = 20MHz
AD7475 3V
SCLK = 20MHz
01684-B-025
Figure 24. Power vs. Throughput for Partial Power Down
For example, if the AD7495 is operated in a continuous
sampling mode with a throughput rate of 100 kSPS and an
SCLK of 20 MHz (V
DD
= 5 V), and the device is placed in partial
power-down mode between conversions, then the power
consumption is calculated as follows. The maximum power
dissipation during normal operation is 13 mW (V
DD
= 5 V). If
the power-up time from partial power-down is one dummy
cycle, that is, 1 μs, and the remaining conversion time is another
cycle, that is, 1 μs, then the AD7495 can be said to dissipate
13 mW for 2 μs during each conversion cycle. For the remainder of
the conversion cycle, 8 μs, the device remains in partial power-
down mode. The AD7495 dissipates 1.15 mW for the remaining
8 μs of the conversion cycle. If the throughput rate is 100 kSPS,
and the cycle time is 10 μs, the average power dissipated during
each cycle is (2/10) × (13 mW) + (8/10) × (1.15 mW) = 3.52 mW. If
V
DD
= 3 V, SCLK = 20 MHz and the device is again in partial
power-down mode between conversions, the power dissipated
during normal operation is 6 mW.
The AD7495 dissipates 6 mW for 2 μs during each conversion
cycle and 0.69 mW for the remaining 8 μs where the device is in
partial power-down. With a throughput rate of 100 kSPS, the
average power dissipated during each conversion cycle is (2/10) ×
(6 mW) + (8/10) × (0.69 mW) = 1.752 mW. Figure 24 shows the
power vs. throughput rate when using partial power-down mode
between conversions with both 5 V and 3 V supplies for both the
AD7475 and AD7495. For the AD7475, partial power-down
current is lower than that of the AD7495.
Full power-down mode is intended for use in applications with
slower throughput rates than required for partial power-down
mode. It is necessary to leave 650 μs for the AD7495 to be fully
powered up from full power-down before initiating a conversion.
Current consumptions between conversions is typically less
than 1 μA.
Figure 25 shows a typical graph of current vs. throughput for
the AD7495 while operating in different modes. At slower
throughput rates, for example, 10 SPS to 1 kSPS, the AD7495
was operated in full power-down mode. As the throughput rate
increased, up to 100 kSPS, the AD7495 was operated in partial
power-down mode, with the device being powered down between
conversions. With throughput rates from 100 kSPS to 1 MSPS,
the device operated in normal mode, remaining fully powered
up at all times.
THROUGHPUT (SPS)
2.0
10
CURRENT (mA)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
100 1k 10k 100k 1M
V
DD
= 5V
FULL
POWER-DOWN
PARTIAL
POWER-DOWN
NORMAL
01684-B-026
Figure 25. Typical AD7495 Current vs. Throughput